Rate adaptive pacemaker using impedance measurements and stroke volume calculations

ABSTRACT

A rate adaptive pacemaker comprises a means ( 2 ) for determining the demand of the patient&#39;s organism, a pacing rate controlling means ( 16 ) for controlling the pacing rate in response to the patient&#39;s demand, and a pacing rate limiting means ( 20 ) for preventing the pacing rate from becoming too low. The pacing rate limiting means is adapted to limit the pacing rate downwards such that a first predetermined relation is satisfied between actual cardiac output (CO) and cardiac output (CO rest ) for the patient in rest conditions and a second predetermined relation is satisfied between actual stroke volume (SV) and rest stroke volume (SV rest ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rate adaptive pacemaker of the typehaving an arrangement for determining the demand of the patient'sorganism, a pacing rate control for controlling the pacing rate inresponse to the patient's demand, and a pacing rate limiter forpreventing the pacing rate from becoming too low.

2. Description of the Prior Art

The pacing rate of a rate adaptive pacemaker may become too low due tothe physical demand of the patient's organism and heart. This may resultin lack of oxygen supply to the myocardium. Under certain conditions theheart may not be able to fulfil the physiological needs of the patient'sorganism and heart if the pacing rate is not limited.

It is known to set a lower limit for the pacing rate. This limit valueis normally determined from the patient's diagnosis and a constant orexternally programmable limit can be set. Thus U.S. Pat. No. 4,535,774describes a stroke volume controlled pacemaker, in which the heart rateis permitted to range between prescribed minimum and maximum heart ratevalues. Further, in U.S. Pat. No. 5,861,011 a pacemaker is disclosedhaving a system for determining the circadian rhythm by examiningvariations in the QT interval and adjusting the pacemaker night timesetting of a lower rate limit to the lower value than the pacemakerdaytime setting of the lower rate limit. In U.S. Pat. No. 5,183,040, anantitachycardia pacer is disclosed which analyzes cardiac outputinformation for assessing hemodynamic status and determining adequateblood supply.

Thus, a pacing rate that is too low may cause an influx of bloodenriched with oxygen that also is too low. A prescribed suitable lowerpacing rate limit avoids the slow influx of the fresh blood. At the sametime this lower limit value should be low enough so as to not to disturba peaceful sleep. In that case the patient can feel more healthy invarious everyday life conditions including peaceful sleeping.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a rate adaptivepacemaker in which the pacing rate is prevented from becoming too low,such that the above discussed inconveniences for the patient areavoided.

The above object is achieved in accordance with the principles of thepresent invention in a rate adaptive pacemaker having an arrangementadapted for interaction with a subject for obtaining an electricalsignal representing cardiac demand of the subject, a computing unitsupplied with the signal for calculating an actual cardiac output CO anda cardiac output CO_(rest) for the subject at rest, a pacing ratecontroller also supplied with the signal for generating, as an output, apacing rate dependent on the cardiac demand, and a pacing rate limiterconnected to the computing unit and to the pacing rate controller fordownwardly limiting the pacing rate, wherein the pacing rate limitercalculates a stroke volume SV and a stroke volume SV_(rest) for thesubject at rest, and downwardly limits the pacing rate so that a firstpredetermined relation CO>CO_(rest) and a second predetermined relationSV/SV_(rest)<L are satisfied, wherein L is a predetermined constant in arange between 1.2 and 1.5.

Thus, by satisfying two predetermined relations the pacemaker accordingto the invention ensures a sufficient minimum energy supply to thepatient's organism or body and at the same time the maximum value of thestroke volume is limited and these conditions are continuouslyautomatically checked.

In one embodiment of the pacemaker 20 according to the invention thefirst predetermined relation isCO>_(Corest)  (1)and the second predetermined relation is(SV)/(SV _(rest))<L  (2)where L denotes a predetermined constant >1, preferably equal to a valuebetween 1.2 and 1.5. In this way it is ensured that the actual cardiacoutput will not become lower than the rest state cardiac outputCO_(rest) as well as ensuring that the actual stroke volume will be lessthan a maximum allowed value equal to L×SV_(rest), where L typically hasa value between 1.2 and 1.5, depending on the health of the patient'smyocardium. By satisfying both these conditions simultaneously aphysiologically well founded heart work management at low work loads isensured.

In another embodiment of the pacemaker according to the invention thepacing rate limiter includes a lower limit setting unit for setting alower limit value for the pacing rate, and a lower limit determiningunit for determining the relation between actual cardiac output (CO) andcardiac output (CO_(rest)) for the patent in rest conditions, and therelation between actual stroke volume (SV) and a rest stroke volume(SV_(rent)) and for calculating a lower pacing rate limit value from therelations for supply to said limit setting unit. The lower limitdetermining unit includes a stroke volume measuring unit for measuringactual stroke volume SV and a comparator for comparing measured actualstroke volume SV with stroke volume SV_(rest) for the patient in restconditions to ensure that the inequalitySV/SV _(rest) <L  (3)is satisfied. The lower limit determining unit is adapted to calculate alower pacing rate limit value from the equationlower pacing rate limit=HR _(rest)(_(Svrest) /SV)  (4)where HR_(rest) denotes the heart rate for the patient in restconditions, provided that said inequality is satisfied. In this way thelower pacing rate limit is continuously automatically calculated and itmay also happen that the lower pacing rate limit becomes lower than thetypical heart rate HR_(rest) for rest conditions of the patient.

In another embodiment of the pacemaker according to the invention abioimpedance measurement unit is provided to measure the cardiacbioimpedance as a function of time for determining therefrom actualcardiac output CO and actual stroke volume SV from the measured cardiacbioimpedance. These parameters thus are obtained in an easy and reliableway from the time variation of the bioimpedance measured between astandard intracardiac electrode and the housing of the pacemaker, whenan excitation current proceeds from the electrode tip.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a rate adaptive pacemakerconstructed and operating in accordance with the principles of thepresent invention.

FIG. 2 illustrates the principle of bioimpedance measurements betweenthe tip of an intracardial electrode and the metallic housing of thepacemaker, these measurements being used in the rate adaptive pacemakerof FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To avoid the current cardiac output COCO=SV×HR  (5)becomes lower than the rest state cardiac output CO_(rest) the pacingrate must be above a lower pacing rate limit given bylower pacing rate limit=(_(Corest))/(SV)  (6)and sinceCO _(rest) =HR _(rest)×_(Svrest)  (7)lower pacing rate limit=(HR _(rest))×(_(Svrest) /SV)  (8)

In addition the maximum value of the stroke volume must be limited, i.e.SV<L× _(Svrest)  (9)

Thus, the following two conditions must be fulfilled simultaneously forinsuring a physiologically well founded heart work management at lowwork loads.Pacing rate limit>(HR _(rest))×(_(Svrest) /SV)  (10)SV/SV _(rest) <L  (11)where L is a constant typically equal to a value of 1.2 to 1.5,depending on the health of the patient's myocardium.

Thus the lower pacing rate limit is continuously automaticallycalculated from the measured actual stroke volume SV and known values ofSV_(rest), HR_(rest) and the constant L. The actual stroke volume can bedetermined from e.g. bioimpedance measurements as will be describedbelow.

FIG. 1 is a block diagram of an embodiment of the pacemaker according tothe invention having a bioimpedance measurement unit 2 for measuring thetime variation of the electric intracardiac bioimpedance Z_(e)(t). Thistype of measurement is well-known, see e.g. “Design of CardiacPacemakers”, edited by John G. Webster, IEEE Press, 1995, pp. 380–386and U.S. Pat. Nos. 5,154,171, 5,280,429, 5,282,840 and 5,807,272. Thusthe time variation of the intracardiac bioimpedance can be measuredbetween the tip 4 of the intracardiac electrode 6 and the housing 8 ofthe pacemaker, when an excitation current is fed from the electrode tip4, as schematically illustrated in FIG. 2. Thus a standard pacing leadcan be used for this measurement.

From the measured time variations AZc(t) the stroke volume SV needed forcalculating the lower pacing rate limit according to equation (8) above,or for checking the inequalities (10) or (11), are determined in acomputing unit 10, see FIG. 1.

The calculated lower limit value is supplied to a lower limit 30 settingunit 12 of a pacing rate limiter 14.

A pacing rate controller 16 is also provided for controlling the pacingrate of the pacer or pulse generator 18 in response to the patient'sdemands. In a limiting unit 20 of the limiter 14 the demanded pacingrate is compared to the set lower limit pacing rate and the actualpacing rate is limited to the set lower limit value if the demandedpacing rate reaches this limit value. Thus in the pacemaker according tothe invention a lower limit value for the pacing rate is continuouslyautomatically determined and it is continuously automatically verifiedthat the actual pacing rate does not exceed the present lower limitvalue.

Alternatively, the pacemaker can be modified to continuously monitorthat the inequalities (10) or (11) above are satisfied.

Above bioimpedance measurements are described for determining the strokevolume SV. This parameter can, however, also be determined by othertechniques, like by ECG measurements, by ultrasound technique, byradiometric and optical techniques etc. Generally all dynamic distanceand/or capacity measuring methods are applicable.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1. A rate adaptive pacemaker comprising: an arrangement adapted forinteraction with a subject for obtaining an electrical signalrepresenting cardiac demand of said subject; a computing unit suppliedwith said signal for calculating an actual cardiac output CO, and acardiac output CO_(rest) for said subject at rest; a pacing ratecontroller also supplied with said signal for generating, as an output,a pacing rate dependent on said cardiac demand; and a pacing ratelimiter connected to said computing unit and to said pacing ratecontroller for downwardly limiting said pacing rate, said pacing ratelimiter calculating a stroke volume SV and a stroke volume SV_(rest) forsaid subject at rest, and downwardly limiting said pacing rate so that afirst predetermined relation CO>CO_(rest) and a second predeterminedrelation SV/SV_(rest)<L are satisfied, wherein L is a predeterminedconstant in a range between 1.2 and 1.5.
 2. A rate adaptive pacemaker asclaimed in claim 1 wherein said pacing rate limiter comprises a lowerlimit setting unit for setting a lower limit value for said pacing rateand a lower limit determining unit wherein SV and SV_(rest) aredetermined, and wherein a relation between CO and CO_(rest) and arelation between SV and SV_(rest) are determined, and for calculating alower pacing rate limit value from said relations for supply to saidlimit setting unit.
 3. A rate adaptive pacemaker as claimed in claim 2wherein said lower limit determining unit includes a comparator forcomparing SV/SV_(rest) to L to ensure that (SV/SV_(rest)<is satisfied,and for calculating said lower pacing rate limit value as a product ofHR_(rest) and Sv_(rest)/SV), wherein HR_(rest) is a heart rate for saidsubject at rest, provided (SV/SV_(rest))<L is satisfied.
 4. A rateadaptive pacemaker as claimed in claim 1 wherein said arrangement fordetermining cardiac demand comprises a bioimpedance measuring unitadapted to measure a cardiac bioimpedance as a function of time as saidsignal representing cardiac demand.
 5. A rate adaptive pacemaker asclaimed in claim 1 wherein said arrangement for determining cardiacdemand comprises an ECG measuring unit for generating an ECG signal assaid signal representing cardiac demand.
 6. A rate adaptive pacemaker asclaimed in claim 1 wherein said arrangement for determining cardiacdemand is a dynamic distance measuring unit which generates a distancesignal as said signal representing cardiac demand.